thp: add compound_trans_head() helper

Cleanup some code with common compound_trans_head helper.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Avi Kivity <avi@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -415,20 +415,11 @@ out:
 static struct page *page_trans_compound_anon(struct page *page)
 {
 	if (PageTransCompound(page)) {
-		struct page *head;
-		head = compound_head(page);
+		struct page *head = compound_trans_head(page);
 		/*
-		 * head may be a dangling pointer.
-		 * __split_huge_page_refcount clears PageTail
-		 * before overwriting first_page, so if
-		 * PageTail is still there it means the head
-		 * pointer isn't dangling.
+		 * head may actually be splitted and freed from under
+		 * us but it's ok here.
 		 */
-		if (head != page) {
-			smp_rmb();
-			if (!PageTransCompound(page))
-				return NULL;
-		}
 		if (PageAnon(head))
 			return head;
 	}